chore: update ci.yml

Author: ExplodingKonjac

.github/workflows/ci.yml

@@ -4,6 +4,9 @@ on:
     branches: ["**"]
   pull_request:
 
+env:
+  RUST_TOOLCHAIN: stable
+
 jobs:
   fmt:
     runs-on: ubuntu-latest
@@ -13,7 +16,7 @@ jobs:
         with:
           components: rustfmt
           toolchain: ${{ vars.RUST_TOOLCHAIN }}
-          
+
       - uses: Swatinem/rust-cache@v2
       - name: cargo fmt
         run: cargo fmt --all -- --check